The 6-pin Serial cable (FTDI cable) is used to access the Linux terminal of the board. If you don’t have it you won't be able to access the terminal. The FTDI cable allows you to configure the board from the Linux terminal. Please check the following guide for further details: IoT - Set Up Serial Terminal on the Intel® Galileo Board for Windows* | Intel® Developer Zone
Regarding the use of the Eclipse IDE, I recommend you to check the following guide: IoT - Installing the Eclipse* IDE | Intel® Developer Zone.
The guide you followed previously (IoT - Step 1: Make a bootable micro SD card | Intel® Developer Zone) is to make a bootable SD card with the IoT image. If you completed all the process then you have the IoT image in your SD card. Then you can continue with the Eclipse IDE installation guide I mentioned above. If you have issues with the IP address, I suggest you to try connecting the board directly to your router.